A fingerprint sensor includes a pixel array, an analog-to-digital converter, and a reference signal generator. The pixel array includes a plurality of unit pixels arranged in rows and columns, and each of the plurality of unit pixels generates an analog signal by detecting a fingerprint of a user. The analog-to-digital converter performs an analog-to-digital conversion operation on the analog signal to generate a digital signal. The analog-to-digital converter is configured to perform the analog-to-digital conversion based on a reference signal. The reference signal generator generates the reference signal such that the reference signal decreases at a first rate from a start voltage, and adjusts a magnitude of the start voltage based on the digital signal.